So I have been curious if he'd act the same way around other dogs and food. So yesterday when I took him to visit my Grandma's 2 dogs, I fed them treats. I pulled 2 treats out, had him and Markus in a sit, and gave them each their treat; it worked just fine the first time. However, I pulled out a second treat for each of them, and before I could even get them in a sit, Oliver gave out a really nasty growl and snarl at Markus and lunged at him. It was pretty scary! This is not behavior that I want to continue.

I used to take him to puppy socials, where the instructor would always have us get the puppy's attention with treats. There was always a dog on either side of Oliver that was also receiving a treat, and he had no problem with it. For some reason this behavior has developed as an adult. But it makes me afraid to bring him to any more training where there will be treats or ever board him if I have to. I'm definitely getting in contact with a trainer next week.

Instead of avoidng I would work on it. Seems that your dog was too close to the others. So I would start at a distance from the other dogs - distance where he doesn't react. Once he appears OK, then reduce the distance. Keep training at that distance as long as he is comfortable with the presence of other dogs. If he reacts, increase the distance again.

Don't give up, if you want a happy dog, comfortable around other dogs, you need to work at it.

I agree! I'm not going to give up, but it's definitely a scary thing to watch. I've never had a dog with a behavioral "issue" like this

I already contacted behaviorists to help me. I don't feel comfortable working on it on my own...